Suppose that, as in Example 9.11, a bank allows 4-digit PINs, but doesn’t permit a PIN that starts with the same digit repeated twice (for example, 7730) or ends with the same digit repeated twice...

Suppose that, as in Example 9.11, a bank allows 4-digit PINs, but doesn’t permit a PIN that starts with the same digit repeated twice (for example, 7730) or ends with the same digit repeated twice (for example, 0122). Now how many invalid PINs are there?